Systems and methods for controlling electric machines

ABSTRACT

A motor controller is provided that includes an inverter configured to drive an electric motor, a rectifier configured to rectify an alternating current (AC) input current and to output the rectified AC input current to the inverter, and a controller coupled to the inverter. The controller is configured to improve a power factor of the motor controller by controlling the AC input current based on a direct current (DC) link voltage measurement.

BACKGROUND

The field of the disclosure relates generally to electric motorcontrollers, and more specifically to methods and a controller forreducing size and costs of motor controllers for electric motors.

Devices commonly known as electronic motor controllers are utilized tocontrol the operation of certain electric motors. At least some knownmotor controllers have attempted to reduce cost and save resources byreplacing large-capacity smoothing capacitors with small-capacitycapacitors. Because of the small-capacity capacitor, a rectified inputvoltage to be applied to an inverter is unable to be properly smoothedand has a pulsating waveform. The voltage of the pulsating waveform hasa frequency about twice that of an output voltage of an alternatingcurrent (AC) power supply to which it is connected.

When using electric motor controllers, a sinusoidal input current may besacrificed, which can lead to a poor power factor for the electricmotor. Active power factor correction devices are known to correct thepower factor, but are typically large in size and are often costly.Alternatively, at least some known motor controllers apply a torquecommand to be synchronous with line input voltage to correct poor powerfactor. However, measuring line input voltage necessitates an additionalisolated voltage sensor, which increases the system cost.

BRIEF DESCRIPTION

In one aspect, a motor controller is provided that includes an inverterconfigured to drive an electric motor, a rectifier configured to rectifyan alternating current (AC) input current and to output the rectified ACinput current to the inverter, and a controller coupled to the inverter.The controller is configured to improve a power factor of the motorcontroller by controlling the AC input current based on a direct current(DC) link voltage measurement.

In another aspect, a method of controlling an electric motor using amotor controller is provided. The method includes controlling an ACinput current based on a DC link voltage measurement to improve a powerfactor of the motor controller.

In yet another aspect, a system is provided that includes an electricmotor, an inverter configured to drive the electric motor, and a motorcontroller coupled to the inverter. The motor controller includes arectifier and is configured to improve a power factor of the motorcontroller by controlling an AC input current based on a DC link voltagemeasurement.

DRAWINGS

FIG. 1 is a block diagram of an exemplary motor control system.

FIG. 2 is a block diagram of an exemplary power factor correctionalgorithm that may be implemented by the motor controller shown in FIG.1.

FIG. 3 is a simplified block diagram of an exemplary sinusoidal waveformgeneration device that may be used by the sinusoidal waveform generatorshown in FIG. 2.

FIG. 4 is a simplified block diagram of an exemplary sinusoidal waveformgeneration device that may be used by the sinusoidal waveform generatorshown in FIG. 2.

FIG. 5 illustrates a waveform chart of input signals achieved using theexemplary power factor correction algorithm shown in FIG. 2.

FIG. 6 is a block diagram of a first alternative power factor correctionalgorithm that may be implemented by the motor controller shown in FIG.1.

FIG. 7 is a block diagram of a second alternative power factorcorrection algorithm that may be implemented by the motor controllershown in FIG. 1.

FIG. 8 illustrates a waveform chart of input signals achieved using thesecond alternative power factor correction algorithm shown in FIG. 7.

FIG. 9 is a block diagram of a third alternative power factor correctionalgorithm that may be implemented by the motor controller shown in FIG.1.

FIG. 10 illustrates a waveform chart of input signals achieved using theexemplary power factor correction algorithm shown in FIG. 9.

FIG. 11 is a block diagram of a fourth alternative power factorcorrection algorithm that may be implemented by the motor controllershown in FIG. 1.

FIG. 12 illustrates a waveform chart of input signals achieved using theexemplary power factor correction algorithm shown in FIG. 11.

FIG. 13 illustrates a flowchart of an exemplary method of operating anelectric motor using a motor controller.

DETAILED DESCRIPTION

The embodiments described herein relate to electric motor controllersand methods of operating the same. More particularly, the embodimentsrelate to a motor controller that eliminates large filter capacitors andmaintains a high power factor for an electric motor. More particularly,the embodiments relate to a motor controller configured to control ACinput current based on based on a direct current (DC) link voltagemeasurement to facilitate improving a power factor of the electricmotor. It should be understood that the embodiments described herein forelectrical machines are not limited to motors, and should be furtherunderstood that the descriptions and figures that utilize a motor areexemplary only. Moreover, while the embodiments illustrate a three phaseelectric motor, the embodiments described herein may be included withinmotors having any number of phases, including single phase and multiplephase electric motors.

FIG. 1 is a block diagram of an electric motor system 100 that includesa motor controller 102. In the exemplary embodiment, electric motorsystem 100 also includes an electric motor 104, an inverter 106 coupledto electric motor 104, and a power supply 108 coupled to motorcontroller 102. In the exemplary embodiment, electric motor 102 includesa permanent magnet synchronous motor. However, any type of electricmotor may be used that enables electric motor system 100 to function asdescribed herein.

In the exemplary embodiment, power supply 108 supplies a single-phasealternating current (AC) voltage to motor controller 102. However, powersupply 108 may supply three-phase AC or any other type of input voltagethat enables electric motor system 100 to function as described herein.

Inverter 106 conditions a pulsed DC voltage received from motorcontroller 102, and supplies it to electric motor 104, where it is useddrive electric motor 104. In the exemplary embodiment, inverter 106converts the pulsed DC voltage to a three-phase AC voltage.Alternatively, inverter 106 converts the pulsed DC voltage to any typeof voltage that enables electric motor system 100 to function asdescribed herein.

In the exemplary embodiment, motor controller 102 includes a rectifier110 configured to rectify an alternating current (AC) input current andto output the rectified AC input current to inverter 106. A controller112, which may sometimes be referred to as a microcontroller/DSP, isprogrammed to control operation of a rotating machine portion (notshown) of electric motor 104. Six pulse width modulated signals areutilized to induce rotation of the rotating machine, via inverter 106,which enables electric motor 104 to be referred to as a three-phasemotor. Signals received from the rotating machine at controller 112include signals relating to the current drawn by each of the phases andan AC input current, or DC bus voltage. Controller 112 is coupled toinverter 106 and is configured to increase a power factor of electricmotor 104 by controlling the AC input current based on a direct current(DC) link voltage measurement, as described in more detail herein.

In some embodiments, motor controller 102 includes a low-capacitancecapacitor 114 for storing small amounts of energy when input voltage isavailable. Capacitor 114 also supplies power to the electronics of motorcontroller 102. In one embodiment, film capacitor 114 has a capacitanceof about 2 μF. Capacitor 114 may have a capacitance between about 0.1μF/kW and about 10 μF/kW. The use of bulky, unreliable electrolyticfilter capacitors in motor controller 102 is avoided.

Motor controller 102 also includes a voltage sensor 116 coupled acrosscapacitor 114. Voltage sensor 116 is configured to measure a DC linkvoltage across capacitor 114. Voltage sensor 116 provides a DC linkvoltage measurement to controller 112 for use in controlling electricmotor 102 to increase a power factor of electric motor by controllingthe AC input current based on the DC link voltage measurement. Morespecifically, controller 112 is configured to implement an algorithmconfigured to increase power factor based on the DC link voltagemeasurement from voltage sensor 116.

FIG. 2 is a block diagram of an exemplary power factor correctionalgorithm 200 that may be implemented by motor controller 102 (shown inFIG. 1). In the exemplary embodiment, controller 102 is configured toimplement the algorithm to determine a q-axis reference value based onmeasured DC link voltage.

In the exemplary embodiment, motor phase currents I_(a), I_(b), andI_(c) are sensed using current sensors 202. An abc-dq converter 204converts the three-phase current values to a two-phase d-q coordinatesystem, giving measured current values I_(d) and I_(q), which are inputinto a d-q axis current controller 206.

A sinusoidal waveform generator 208 receives a measured DC link voltagefrom voltage sensor 116 (shown in FIG. 1) and generates a sinusoidalwaveform. The sinusoidal waveform is multiplied by a current referencesignal I_(q)* at multiplier 210. I_(q)* represents a torque component ofcurrent. The resulting I_(q)* current reference is input into d-q axiscurrent controller 206. The I_(q)* has twice the line frequency (i.e.,100 Hz or 120 Hz), and is in phase (or slightly advanced/delayed) withinput line voltage from power supply 108 (shown in FIG. 1).

The I_(q)* and I_(d)* (flux-linkage component of current) referencesignals are input into d-q axis current controller 206. D-q axis currentcontroller 206 processes the I_(q)* and I_(d)* reference signals withthe measured current values I_(d) and I_(q). D-q axis current controller206 outputs voltage reference signals u_(q) and u_(d). Voltage referencesignals u_(q) and u_(d) are converted back into three-phase values by adq-abc converter 212. The three-phase voltage reference signals u_(q)and u_(d) are input into a pulse width modulator (PWM) 214, which has asix-step transformation with six outputs that drive inverter 106. Insome embodiments, the signal output by PWM 214 is limited by a dutycycle limiter 216 before being transmitted to inverter 106.

In the exemplary embodiment, electric motor 104 is controlled based onthe availability of power. More specifically, controller 112 receivesthe measured DC link voltage from voltage sensor 116 and outputs thesinusoidal waveform. This waveform is multiplied by torque commandI_(q)* to become the q-axis current reference I_(q)*. Accordingly,controller 112 controls AC input current based on the DC link voltagemeasurement, while increasing the power factor of electric motor 104.

FIG. 3 is a simplified block diagram of an exemplary sinusoidal waveformgeneration device 300 that may be used by the sinusoidal waveformgenerator shown in FIG. 2. In the exemplary embodiment, sinusoidalwaveform generation device 300 includes a low pass filter 302 coupled toa phase compensator 304.

DC link voltage measured by voltage sensor 116 (shown in FIGS. 1 and 2)is input into low pass filter 302. Low pass filter 302 is configured tohave a cut-off frequency lower than a switching frequency of inverter106 (shown in FIGS. 1 and 2), but higher than twice the line frequency(100 Hz or 120 Hz). Such configuration maintains the 100 Hz or 120 Hzfrequency component in the DC link voltage. Low pass filter 302 filtersout a high frequency component of the DC link voltage and transmits itto phase compensator 304.

In some instances, the filtering by low pass filter 302 causes a phasedelay, so phase compensator 304 is provided to correct the phase of theDC link voltage. Once the phase is corrected, the sinusoidal waveform issent to multiplier 210 (shown in FIG. 2) to generate the q-axisreference.

FIG. 4 is a simplified block diagram of an exemplary sinusoidal waveformgeneration device 400 that may be used by the sinusoidal waveformgenerator shown in FIG. 2. In the exemplary embodiment, sinusoidalwaveform generation device 400 includes a phase detector 402 coupled toa sinusoidal generator 404.

DC link voltage measured by voltage sensor 116 (shown in FIGS. 1 and 2)is input into phase detector 402. Phase detector 402 implements one ormore phase detection algorithms to determine the phase of the measuredDC link voltage. The algorithms may include a phase lock loop and/or azero crossing point detection method.

Sinusoidal generator 402 uses the phase of the measured DC link voltageto generate a sinusoidal waveform. The sinusoidal waveform is then sentto multiplier 210 (shown in FIG. 2) to generate the q-axis reference.

FIG. 5 illustrates a waveform chart of input signals achieved using theexemplary power factor correction algorithm shown in FIG. 2. In theexemplary embodiment, AC line voltage 500 is at 60 Hertz (Hz.). AC linecurrent 502 has a quasi-sinusoidal shape and is in-phase orsubstantially in-phase with AC line voltage 500. Because I_(q) ismodulated near 120 Hz, AC line current 502 is forced to follow thesinusoidal shape of AC line voltage 500, leading to a higher powerfactor for motor controller 102 (shown in FIG. 1).

FIG. 6 is a block diagram of a first alternative power factor correctionalgorithm 600 that may be implemented by motor controller 102 (shown inFIG. 1). In the exemplary embodiment, controller 102 is configured toimplement the algorithm to smooth the DC link voltage before generatinga pulse width modulation (PWM) signal.

In the exemplary embodiment, motor phase currents I_(a), I_(b), andI_(c) are sensed using current sensors 602. An abc-dq converter 604converts the three-phase current values to a two-phase d-q coordinatesystem, giving measured current values I_(d) and I_(q), which are inputinto a d-q axis current controller 606.

Current reference values I_(d)* and I_(q)* are input into d-q axiscurrent controller 606. D-q axis current controller 606 processes theI_(q)* and I_(d)* reference signals with the measured current valuesI_(d) and I_(q). D-q axis current controller 606 outputs voltagereference signals u_(q) and u_(d). Voltage reference signals u_(q) andu_(d) are converted back into three-phase values by a dq-abc converter608. The three-phase voltage reference signals u_(q) and u_(d) are inputinto a PWM generator 610, which has a six-step transformation with sixoutputs that drive inverter 106.

A voltage smoother 612 is coupled between voltage sensor 116 and PWMgenerator 610. Because DC link voltage is needed for PWM duty ratiocalculation, the switching harmonics in the DC link voltage may beintroduced to PWM duty ratio, causing resonance issues. Voltage smoother612 smoothes the DC link voltage and avoids such resonance issues, whichimproves the power factor of motor controller 102 and improves thecurrent waveform of electric motor 104. In one embodiment, voltagesmoother 612 includes sinusoidal waveform generation device 300 (shownin FIG. 3), including low pass filter 302 and phase compensator 304. Inanother embodiment, voltage smoother 612 includes sinusoidal waveformgeneration device 400 (shown in FIG. 4), including phase detector 402and sinusoidal generator 404. PWM generator 610 then outputs three-phasevoltage command signals V_(a), V_(b), and V_(c) to a duty cycle limiter614.

FIG. 7 is a block diagram of a second alternative power factorcorrection algorithm 700 that may be implemented by motor controller 102(shown in FIG. 1). In the exemplary embodiment, controller 102 isconfigured to implement the algorithm to control a duty cycle of atleast one voltage commanded by said controller.

In the exemplary embodiment, motor phase currents I_(a), I_(b), andI_(c) are sensed using current sensors 702. An abc-dq converter 704converts the three-phase current values to a two-phase d-q coordinatesystem, giving measured current values I_(d) and I_(q), which are inputinto a d-q axis current controller 706.

Current reference values I_(d)* and I_(q)* are input into d-q axiscurrent controller 706. D-q axis current controller 706 processes theI_(q)* and I_(d)* reference signals with the measured current valuesI_(d) and I_(q). D-q axis current controller 706 outputs voltagereference signals u_(q) and u_(d). Voltage reference signals u_(q) andu_(d) are converted back into three-phase values by a dq-abc converter708. The three-phase voltage reference signals u_(q) and u_(d) are inputinto a PWM generator 710, which has a six-step transformation with sixoutputs that drive inverter 106. PWM generator 710 then outputsthree-phase voltage command signals V_(a), V_(b), and V_(c) to a dutycycle limiter 712.

Duty cycle limiter 712 employs different types of limiting functions 714around the zero crossings of the AC line current, including but notlimited to, linear and/or polynomial limiting functions. In theexemplary embodiment, duty cycle limiter 712 uses a sinusoidal limitingfunction derived and/or synchronized from AC power source 108. Afterapplying limiting function 714 to voltage command signals V_(a), V_(b),and V_(c), duty cycle limiter 712 outputs modulated voltage referencesignals V_(a)*, V_(b)*, and V_(c)* to inverter 106 for driving electricmotor 104.

FIG. 8 illustrates a waveform chart of input signals achieved using thesecond alternative power factor correction algorithm shown in FIG. 7.Controlling the duty cycle includes controlling the AC line current 800to shape it near the vicinity of the zero crossings of the AC sourcevoltage 802. By doing so, sharp transitions 804 in AC line current 800are smoothed and the power factor is improved, as can be seen in theduty cycle limiting enabled graph. Duty cycle limiter 712 also assistsin the prevention of oscillations in the AC line current (or busvoltage) by gradual regulation of the load flow independently of theoutput swings in d-q axis current controller 706.

FIG. 9 is a block diagram of a third alternative power factor correctionalgorithm 900 that may be implemented by motor controller 102 shown inFIG. 1. In the exemplary embodiment, controller 102 is configured toimplement an algorithm to increase the power factor based on the DC linkvoltage measurement and an AC line current measurement.

In the exemplary embodiment, motor phase currents I_(a), I_(b), andI_(c) are sensed using current sensors 902. An abc-dq converter 904converts the three-phase current values to a two-phase d-q coordinatesystem, giving measured current values I_(d) and I_(q), which are inputinto a d-q axis current controller 906.

A current reference generator 908 receives a measured DC link voltagefrom voltage sensor 116 (shown in FIG. 1) and generates a sinusoidal ACcurrent reference signal I_(AC)*. Additionally, AC line current I_(AC)is measured using a current sensor 910. I_(AC) is subtracted fromI_(AC)* in summing junction 912. The resultant signal is multiplied by atorque command I_(q)*′ at multiplier 914. The resulting I_(q)* currentreference is input into d-q axis current controller 906.

Current reference values I_(d)* and I_(q)* are input into d-q axiscurrent controller 906. D-q axis current controller 906 processes theL_(i)* and I_(d)* reference signals with the measured current valuesI_(d) and I_(q). D-q axis current controller 906 outputs voltagereference signals u_(q) and u_(d). Voltage reference signals u_(q) andu_(d) are converted back into three-phase values by a dq-abc converter916. The three-phase voltage reference signals u_(q) and u_(d) are inputinto a PWM generator 918, which has a six-step transformation with sixoutputs that drive inverter 106. PWM generator 918 then outputsthree-phase voltage command signals V_(a), V_(b), and V_(c) to a dutycycle limiter 920. Duty cycle limiter 920 uses the values inputted toadjust the duty cycle of the AC line current so as to optimize the powerflow to achieve sinusoidal AC input line current.

FIG. 10 illustrates a waveform chart of input signals achieved using thethird alternative power factor correction algorithm shown in FIG. 9. Asshown in FIG. 10, AC line voltage 1000 and AC line current 1002 arein-phase, or substantially in-phase to achieve a near unity powerfactor. The near unity power factor is accomplished by sensing AC inputcurrent and controlling or reconstructing it to have a sinusoidalwaveform to match the AC input voltage.

FIG. 11 is a block diagram of a fourth alternative power factorcorrection algorithm 1100 that may be implemented by motor controller102 (shown in FIG. 1). In the exemplary embodiment, controller 102 isconfigured to implement an algorithm to increase the power factor basedon the DC link voltage measurement and an AC line current measurement,and adjust a harmonic content of electric motor 104 (shown in FIG. 1).

In the exemplary embodiment, motor phase currents I_(a), I_(b), andI_(c) are sensed using current sensors 1102. An abc-dq converter 1104converts the three-phase current values to a two-phase d-q coordinatesystem, giving measured current values I_(d) and I_(q), which are inputinto a d-q axis current controller 1106.

A current reference generator 1108 receives a measured DC link voltagefrom voltage sensor 116 (shown in FIG. 1) and generates a fundamental ACcurrent reference signal I_(AC) _(—) _(fundamental)*. Additionally, aharmonic generation unit 1110 generates a harmonic signal I_(AC) _(—)_(harmonics)*. I_(AC) _(—) _(fundamental)* and I_(AC) _(—) _(harmonics)*are summed at first summing junction 1112 to form an AC currentreference signal I_(AC)*. Modification of the harmonic content enablesmotor controller 102 to achieve an improved power factor for electricmotor 104 (shown in FIG. 1).

Additionally, AC line current I_(AC) is measured using a current sensor1114. I_(AC) is subtracted from I_(AC)* in second summing junction 1116.The resultant signal is multiplied by a torque command I_(q)*′ atmultiplier 1118. The resulting I_(q)* current reference is input intod-q axis current controller 1106.

Current reference values I_(d)* and I_(q)* are input into d-q axiscurrent controller 1106. D-q axis current controller 1106 processes theI_(q)* and I_(d)* reference signals with the measured current valuesI_(d) and I_(q). D-q axis current controller 1106 outputs voltagereference signals u_(q) and u_(d). Voltage reference signals u_(q) andu_(d) are converted back into three-phase values by a dq-abc converter1120. The three-phase voltage reference signals u_(q) and u_(d) areinput into a PWM generator 1122, which has a six-step transformationwith six outputs that drive inverter 106. PWM generator 1122 thenoutputs three-phase voltage command signals V_(a), V_(b), and V_(c) to aduty cycle limiter 1124. Duty cycle limiter 1124 uses the valuesinputted to adjust the duty cycle of the AC line current so as tooptimize the power flow to achieve sinusoidal AC input line current.

FIG. 12 illustrates a waveform chart of input signals achieved using theexemplary power factor correction algorithm shown in FIG. 11. In theexemplary embodiment, the waveform chart includes AC line voltage 1200and AC line current 1202 when motor controller 102 (shown in FIG. 1)implements algorithm 1100. In the exemplary embodiment, the harmoniccontrol algorithm 1100 enables power factor correction using AC linecurrent measurement with seventh harmonic injection. However, any orderharmonic injection may be used that enables motor controller 102 tofunction as described herein. When a wave form that does not have theseventh harmonic is desired, the seventh harmonic is either added orsubtracted from the current reference I_(AC) _(—) _(fundamental)*. Thissummation enables modification of the shape of the AC line current 1202by injecting harmonics into the current wave form. The injectedharmonics alter the wave form of AC line current 1202 such that itsphase becomes close to being the phase of AC line voltage 1200,resulting in an increased power factor. The increased power factor isaccomplished by sensing the AC input current, and controlling orreconstructing it using harmonic control to improve power flow frompower source 108 to electric motor 104.

FIG. 13 is a flowchart 1300 of an exemplary method of controlling anelectric motor using a motor controller. In the exemplary embodiment,the method includes controlling 1302 an AC input current based on a DClink voltage measurement to improve a power factor of the electricmotor. For example, controlling the AC input current includesimplementing 1304 an algorithm to determine a q-axis reference valuebased on the measured DC link voltage. More specifically, in oneembodiment, determining the q-axis reference value includes measuringthe DC link voltage using a voltage detector, processing the measured DClink voltage using a low-pass filter having a cut-off frequency lowerthan a switching frequency of said inverter and higher than twice the ACline frequency, phase shifting the processed DC link voltage, andmultiplying the phase shifted voltage by a torque command. In analternative embodiment, determining the q-axis reference value includesmeasuring the DC link voltage using a voltage detector, processing themeasured DC link voltage using a phase detector, generating asinusoidal-shaped waveform based on the measured phase, and multiplyingthe sinusoidal-shaped waveform by a torque command.

In another embodiment, controlling the AC input current includesimplementing 1306 an algorithm to smooth the DC link voltage beforegenerating a pulse width modulation (PWM) signal.

In yet another embodiment, controlling the AC input current includesimplementing 1308 an algorithm to control a duty cycle of at least onevoltage commanded by the controller.

A technical effect of the systems and methods described herein includesat least one of: (a) controlling an AC input current based on a DC linkvoltage measurement to improve a power factor of the motor controller;(b) implementing an algorithm to determine a q-axis reference valuebased on the measured DC link voltage; (c) implementing an algorithm tosmooth the DC link voltage before generating a PWM signal; and (d)implementing an algorithm to control a duty cycle of at least onevoltage commanded by the controller.

The embodiments described herein relate to electrical motor controllersand methods of operating the same. More particularly, the embodimentsrelate to a motor controller that eliminates large filter capacitors andprovides a high power factor. More particularly, the embodiments relateto a motor controller configured to control AC input current based onbased on a direct current (DC) link voltage measurement to facilitateimproving a power factor of the motor controller. The size rangesdisclosed herein include all the sub-ranges therebetween. The methodsand systems are not limited to the specific embodiments describedherein, but rather, components of systems and/or steps of the methodsmay be utilized independently and separately from other componentsand/or steps described herein. For example, the methods may also be usedin combination with other manufacturing systems and methods, and are notlimited to practice with only the systems and methods as describedherein. Rather, the exemplary embodiment can be implemented and utilizedin connection with many other electrical component applications.

Although specific features of various embodiments of the invention maybe shown in some drawings and not in others, this is for convenienceonly. In accordance with the principles of the invention, any feature ofa drawing may be referenced and/or claimed in combination with anyfeature of any other drawing.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A motor controller comprising: an inverterconfigured to drive an electric motor; a rectifier configured to rectifyan alternating current (AC) input current and to output the rectified ACinput current to said inverter; and a controller coupled to saidinverter and configured to improve a power factor of the motorcontroller by controlling the AC input current based on a direct current(DC) link voltage measurement.
 2. A motor controller in accordance withclaim 1, wherein said controller is configured to implement an algorithmconfigured to increase power factor based on the DC link voltagemeasurement.
 3. A motor controller in accordance with claim 2, whereinsaid controller is configured to implement the algorithm to determine aq-axis reference value based on the measured DC link voltage.
 4. A motorcontroller in accordance with claim 3, wherein to determine the q-axisreference value, said controller is configured to: measure the DC linkvoltage using a voltage detector; process the measured DC link voltageusing a low-pass filter having a cut-off frequency lower than aswitching frequency of said inverter and higher than twice the AC linefrequency; phase shift the processed DC link voltage; and multiply thephase shifted voltage by a torque command.
 5. A motor controller inaccordance with claim 3, wherein to determine the q-axis referencevalue, said controller is configured to: measure the DC link voltageusing a voltage detector; process the measured DC link voltage using aphase detector; generate a sinusoidal-shaped waveform based on themeasured phase; and multiply the sinusoidal-shaped waveform by a torquecommand.
 6. A motor controller in accordance with claim 5, wherein thephase detector implements one of a phase lock loop algorithm and a zerocrossing point detection algorithm.
 7. A motor controller in accordancewith claim 2, wherein said controller is configured to implement thealgorithm to smooth the DC link voltage before generating a pulse widthmodulation (PWM) signal.
 8. A motor controller in accordance with claim7, wherein said controller is configured to use the smoothed DC linkvoltage to avoid resonance in the generation of the PWM signal.
 9. Amotor controller in accordance with claim 2, wherein said controller isconfigured to implement the algorithm to control a duty cycle of atleast one voltage commanded by said controller.
 10. A motor controllerin accordance with claim 9, wherein said controller is configured tocontrol the duty cycle near a zero crossing of an AC input voltage. 11.A motor controller in accordance with claim 1, wherein said controlleris configured to implement an algorithm to increase the power factorbased on the DC link voltage measurement and an AC line currentmeasurement.
 12. A motor controller in accordance with claim 11, furthercomprising an AC line current sensor for measuring AC line current. 13.A motor controller in accordance with claim 11, wherein said controlleris configured to: measure the AC line current; and adjust a duty cycleof the electric motor to improve power flow and generate a sinusoidalinput current based on the measured AC line current.
 14. A motorcontroller in accordance with claim 11, wherein said controller isconfigured to: measure the AC line current; and adjust a harmoniccontent of the AC line current by injecting harmonics into a q-axisreference value.
 15. A motor controller in accordance with claim 1,wherein said controller is configured to increase the power factor ofthe motor controller without sensing at least one of AC line voltage andAC line current.
 16. A motor controller in accordance with claim 1,further comprising a capacitor coupled between said rectifier and saidinverter, said capacitor having a capacitance between about 0.1 μF/kWand about 10 μF/kW.
 17. A method of controlling an electric motor usinga motor controller, said method comprising controlling an alternatingcurrent (AC) input current based on a direct current (DC) link voltagemeasurement to improve a power factor of the motor controller.
 18. Amethod in accordance with claim 17, wherein controlling the AC inputcurrent comprises implementing an algorithm to determine a q-axisreference value based on the measured DC link voltage.
 19. A method inaccordance with claim 18, wherein determining the q-axis reference valuecomprises: measuring the DC link voltage using a voltage detector;processing the measured DC link voltage using a low-pass filter having acut-off frequency lower than a switching frequency of said inverter andhigher than twice the AC line frequency; phase shifting the processed DClink voltage; and multiplying the phase shifted voltage by a torquecommand.
 20. A method in accordance with claim 18, wherein determiningthe q-axis reference value comprises: measuring the DC link voltageusing a voltage detector; processing the measured DC link voltage usinga phase detector; generating a sinusoidal-shaped waveform based on themeasured phase; and multiplying the sinusoidal-shaped waveform by atorque command.
 21. A method in accordance with claim 17, whereincontrolling the AC input current comprises implementing an algorithm tosmooth the DC link voltage before generating a pulse width modulation(PWM) signal.
 22. A method in accordance with claim 17, whereincontrolling the AC input current comprises implementing an algorithm tocontrol a duty cycle of at least one voltage commanded by thecontroller.
 23. A system comprising: an electric motor; an inverterconfigured to drive said electric motor; and a motor controller coupledto said inverter, said motor controller comprising a rectifier, saidmotor controller configured to improve a power factor of the motorcontroller by controlling an alternating current (AC) input currentbased on a direct current (DC) link voltage measurement.
 24. A system inaccordance with claim 23, wherein said controller is configured toimplement an algorithm to determine a q-axis reference value based onthe measured DC link voltage.
 25. A system in accordance with claim 23,wherein said controller is configured to implement the algorithm tosmooth the DC link voltage before generating a pulse width modulation(PWM) signal.
 26. A system in accordance with claim 23, wherein saidcontroller is configured to implement the algorithm to control a dutycycle of at least one voltage commanded by said controller.
 27. A systemin accordance with claim 23, wherein said controller is configured toimplement an algorithm to increase the power factor based on the DC linkvoltage measurement and an AC line current measurement.
 28. A system inaccordance with claim 23, wherein said controller is configured to:measure the AC line current; and adjust a duty cycle of the electricmotor to improve power flow and generate a sinusoidal input currentbased on the measured AC line current.
 29. A system in accordance withclaim 23, wherein said controller is configured to: measure the AC linecurrent; and adjust a harmonic content of the AC line current byinjecting harmonics into a q-axis reference signal.